1. Field of the Invention
The present invention relates to a toner for use in a developer for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like, and a method for manufacturing the same. More particularly, the present invention relates to a toner for electrophotography utilized in copy machines, laser printers and normal paper facsimiles using the direct or indirect electrophotography developing method, and a method for manufacturing the same. Also, the present invention relates to a toner for electrophotography used in full color copy machines, full color laser printers, full color normal paper facsimiles, and the like using the direct or indirect electrophotographic multi-color image developing method, a method for manufacturing the same, and a method for developing using the toner.
2. Description of the Related Art
Developers used in electrophotography, electrostatic recording, electrostatic printing and the like are developed by a process for developing. For example, a developer is adhered to an image carrier such as a photoconductor upon which an electrostatic image is formed, transferred from the photoconductor to a transferring medium such as a transferring paper by the transferring process and fixed on the paper by the fixing process. As the developer for developing the electrostatic image formed on the surface carrying a latent image, two-component developer comprising a carrier and a toner and one-component developer which does not require a carrier such as magnetic toner and non-magnetic toner are known.
Conventionally, dry toners which are used in electrophotography, electrostatic recording and electrostatic printing are prepared by melt-kneading a toner binder such as styrene resins, polyester and the like along with a colorant, followed by pulverization.
In order to obtain an image with high dignity and high quality, it has been attempted to reduce the particle diameter of toners. However, since the toner particles produced by the conventional admixing or roll-mixing, the pulverization method have indefinite shapes and are mixed with ultrafine particles, and, even though a classification process is applied, it still has a problem not to sufficiently classify toner particles as adhered to the toner with the desired particle diameter, because the ultrafine particles are highly adhesive.
The toner particles having indefinite shapes can also be further pulverized by stirring along with a carrier in a developing unit of an image forming apparatus, or by the contact stress with a developing roller, a toner supplying roller, a layer thickness regulating blade or a friction charging blade, when used as a one-component developing agent, thereby producing ultrafine particles. These ultrafine particles can be adhered or fixed on a carrier or mechanical elements, causing problems. Also, fluidity-imparting agents can be embedded in the toner surface, thereby causing deterioration in the image quality.
Also, the toner particles having indefinite shapes are poor in fluidity as powders and filling capability into a toner container and may need a large amount of a fluidity-imparting agent and impede with the compactness of the toner particles in the toner container. Therefore, the advantage of the small diameter particle cannot effectively utilized. Further, there is a limit in the preparation by the pulverization and thus, it is not possible to provide smaller diameter particles.
In addition, since the process for transferring the image formed by multi-color toner from a photoconductor to a transferring medium or paper in order to form a full color image is complex and the indefinite shape of the pulverized toner particles lead to poor transferring property, image omission may occur and a large amount of toner should be used to compensate such image omission.
Therefore, there are demands for prevention of durability from deterioration of ultrafine particles, production of high quality image without image omission and enhancement of transferring efficiency to lower the running cost by reducing the amount of the used toner. If the transferring efficiency is high, a cleaning unit for removing non-transferred toner from a photoconductor or transferring medium is not needed, the small-size apparatus and low cost can be realized, and waste toner can be reduced. In order to solve these problems caused by the indefinite shape of toner particles, various methods for producing a spherical toner have been researched and suggested.
Methods for producing a toner by suspension polymerization have problems in that only spherical toner particles are produced, apparatuses for removing remaining toner particles on a photoconductor or transferring medium (e.g., cleaning blade or cleaning brush) cannot sufficiently remove the spherical toner particles, causing cleaning failure. Also, due to the spherical shape, the toner surface is exposed, in all directions, to contact with charging members such as a carrier or charging blade, added materials and a charge controlling agent existing in the outer surface can be embedded in the toner surface, and thereby, the fluidity of the toner is deteriorated.
Also, in the methods for producing a toner by suspension polymerization, since the toner particles are subjected under irregular shear during the suspension in an aqueous medium, ultrafine particles may be formed, deteriorate cleaning property and adhere to a carrier or mechanical elements.
Meanwhile, methods for producing a toner by emulsion polymerization have problems in that the thermal post-treatment for shaping is needed to produce spherical toner particles and non-congregated ultrafine particles may remain after cohesion and congregation in aqueous medium, which may cause contamination of the carrier and adhesion to mechanical elements.
In order to solve such problems, Japanese Patent Application Laid-Open (JP-A) No. 07-152202 discloses a method accompanying volume contraction, called as polymer dissolution and suspension method of polymer. In this method, a toner material is dispersed and dissolved in a volatile solvent such as low boiling point organic solvent and emulsified and dropped in an aqueous medium containing a dispersing agent, followed by removing of the volatile solvent. At this point, volume contraction occurs. However, when a solid fine particle-dispersing agent which is not dissolved in the aqueous medium is selected as a dispersing agent, indefinite shaped particles are obtained. Also, when solid contents in the solvent is high in order to productivity, the viscosity of the dispersion phase is increased, whereby the particle diameter is increased and particle distribution is broaden. In contrast, when the viscosity of the dispersion phase is reduced by reducing the molecular weight of a resin, image-fixing properties (particularly, hot offset resistance) was compromised.
Japanese Patent Application Laid-Open (JP-A) No. 11-149179 uses a resin of low molecular weight in the dissolution and suspension method to reduce the viscosity of the dispersion phase and to facilitate emulsification and makes the polymerization occur within the particles to improve the image-fixing properties. However, the improvement of the transferring property or cleaning property is not based on the regulation of the shapes of particles.
Also, since the toner particles are subjected under irregular shear during the suspension in an aqueous medium, ultrafine particles may still be formed, deteriorate cleaning property and adhere to a carrier or mechanical parts.
Further, fixing of this dry toner is performed by heating with a hot roll after developing and transferring. At this point, if the temperature of the hot roll is too high, the toner is excessively melted and adhered to the hot roll (hot offset). If the temperature of the hot roll is too low, the toner is not sufficiently melted, causing fixing failure on the roll. In the term of the energy saving and realization of a small size apparatus, there is needed a toner having a high hot offset temperature (hot offset resistance) and low fixing temperature (low temperature image-fixing properties). Also, the storage stability under heat is needed so that the toner is not blocked at a temperature in the apparatus during storage.
In full color copiers and full color printer, glossiness and color miscibility are demanded. Therefore, toners should have a low melt viscosity and a sharp melting polyester-based toner binder is used. However, since such toners tend to cause hot offset, the hot roll in the conventional full color apparatus is coated with silicone oil. In order to perform coating of the hot roll with silicone oil, an oil tank and oil coating apparatus are needed, whereby the apparatus becomes bigger and more complex. Also, the hot roll may deteriorate and maintenance is periodically needed for operation. Further, oil contaminates copy papers, films for OHP (overhead projector), and particularly, discoloration by oil may occur in OHP.
There has not been suggestions to use a solid fine particle-dispersing agent which critically affects toner formation while focusing on the volume contraction by the solid fine particle-dispersing agent.